This invention relates to a photosensitive element for the production of a lithographic printing plate by utilizing the silver complex diffusion transfer process and to a method for preparing such a printing plate.
A lithographic printing plate comprised oleophilic image areas receptive to a greasy ink and oleophilic non-image areas nonreceptive to the ink, said non-image areas being generally made hydrophilic to receive water. Accordingly, the common lithographic printing is performed by feeding both ink and water to the plate surface to allow the image areas to receive preferentially the coloring ink and the non-image areas preferentially the water, and transferring the ink on the image areas to a substrate material such as, for example, paper. In order to produce good prints, it is necessary that both the oleophilicity of image areas and the hydrophilicity of non-image areas be sufficiently high so that when an ink and water are fed to the printing plate surface, the image areas may receive a sufficient amount of the ink and the non-image areas may receive entirely no ink.
The disadvantages accompanying the printing plate produced by the silver complex diffusion transfer process (DTR process) include comparatively rapid deterioration in the endurance of printing ability (printing endurance) owing to an insufficient resistance of the image area against mechanical wear which causes disappearance or gradual deterioration of ink-receptivity of the hydrophobic areas carrying an inked image; build-up of the ink in non-image areas or background owing to gradual sensitization of hydrophilic areas; and deterioration of uniform ink-receptivity of hydrophobic areas, which results in fluctuation of the amount of ink held by the image areas and, hence, causes non-uniformity of the ink-receptivity in printing areas.
Printing plates which utilize as the ink-receptive areas a metallic silver pattern formed on nuclei layer by DTR process are already known [for example, U.S. Pat. Nos. 3,220,837 and 3,721,559; Japanese Patent Publication Nos. 16,725/73 and 30,562/73; Japanese Patent Application "Kokai" (Laid-open) Nos. 4,482,71 and 21,602/78]. Although some of them are in actual use, the above noted problems still remain unsolved.
As is the case with other lithographic printing plates, the lithographic printing plate utilizing DTR process is also required to be improved in printing endurance by improving the construction of printing plate composites, composition of processing solutions, and printing conditions. To meet the requirement, various attempts have heretofore been disclosed. Aside from the ink-receptivity of transferred silver grains precipitated on the nuclei layer, one of the principal factors which affect the printing endurance is loss of transferred silver in the image areas composed of thin lines.
One of the factors controlling the resistance of transferred silver to mechanicl wear is the type of solvent for silver halides as disclosed in Japanese Patent Application "Kokai" (Laid-open) Nos. 146,452/80, 1,057/81, 6,237,81, 8,145/81, 9,749/81 and 9,750/81. Another factor is the degree of hardening with hardeners, such as formaldehyde of silver halide emulsion layer or undercoat layer disposed beneath the physical development nuclei layer. As is well known, with the increase in pH, the hardening of gelatin reaches a higher level in a shorter period of time. The Patent Applications cited above tell that after having been applied, the undercoat layer and the silver halide emulsion layer containing formaldehyde can be hardened by heating at 40.degree. C. for 3 days. In spite of the level of hardening sufficiently high to meet the requirement for a printing plate, which can be attained by heating, the printing plate thus treated was found to have still a disadvantage in that the transerred silver in the image area composed of thin lines still retains the tendency of wearing off during the printing operation, often rendering the printing plate unsuitable for further printing. The present inventors considered that although the hardening level of gelatin is an important factor affecting the printing endurance, there might exist other participating factors and studied the problem from various angles. It was found as anticipated that the aforementioned defect is more enhanced with the decrease in the amount of silver per unit area, particularly when the silver content becomes below 0.8 mmole/m.sup.2 of silver halide in terms of silver.
As stated previously, the printing endurance is diminished by the build-up of ink in non-image areas due to decreased hydrophilicity. The lithographic printing plate should withstand printing over an extended period of time without image loss or scumming. There exists a demand for the development of a lithographic printing plate with more improved printing endurance, though it is a difficult task to maintain two opposite properties of oleophilicity and hydrophilicity simultaneously at satisfying levels throughout the run over an extended period of time.